feat(web): persist language preference

[seekskyworld]

Problem

Implements the language-preference slice of #350. The Web workbench selected a locale only from the browser and lost the choice on reload.

Value

Users can keep a deliberate English/Chinese preference across reloads without involving Session or credential storage.

Approach

Read a validated openpi.language value from browser-local storage, fall back to navigator detection, and add an accessible language toggle. Invalid values are ignored and storage failures degrade to in-memory behavior.

Validation

• npx tsc --noEmit
• node --test --experimental-strip-types tests/web/app-render.test.ts
• git diff --check

Impact

• User-visible behavior: adds a language toggle and reload persistence.
• Model-visible context/tools: none.
• Runtime/lifecycle: none.
• Persisted config/data: browser-local language key only; no Session data or secrets.
• Compatibility/risk: unsupported storage values fall back to browser detection.